Apple did not respond to Business Insider's repeated requests for comment. Digital Mind Co did not immediately respond to a request for comment.

How Calculator% works

The Calculator% on Apple.
Apple

The Calculator% app is marketed on the App Store with pictures of women in bikinis, hardly disguising its purpose to hide photos that are perhaps intimate in nature.

Users download the app and its icon looks like an innocuous calculator. Once open, users can input a secret code into the calculator and it opens a secret vault of images and video concealed to anyone without access.

There are imitations

Digital Mind Co has developed similar apps, including one that acts as a private browser. But like Calculator%, they no longer appear to be on the App Store. Digital Mind Co's calculator apps are not available on Android either.

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Apple appears to have removed imitations as well, such as Calculator App Lock and Piano Pass, which asks users to play a secret tune to unlock their images. Some imitations are still on Android, however, including Calculator+ and Calculator Vault.

Durham Police would not disclose any details about its investigation, but a spokeswoman told Business Insider: "The Calculator app is essentially a secret photo album that children can use to hide photos or videos from their parents. The photos are hidden from view behind an icon that looks like a calculator. We would urge parents to be vigilant and discourage their children from using such an app."